Most of the timber harvest that has occurred on the Tongass National Forest (TNF) has been through clear cutting. The clear-cuts have converted approximately 450,000 acres of Tongass old growth (OG) to even-aged young growth (YG) stands. About half of this YG is in the Suitable and Available timber base, and the rest is in non-timber designated areas.
Approximately 180,000 YG acres have been pre-commercially thinned (PCT). Most of the PCT has been through a variety of types of spaced thinning. Fourteen feet (14′ X 14′) is the preferred spacing between trees for optimum timber production. PCT is usually prescribed when the YG stand is 15 – 25 years old. This is the age when the crowns of the trees are beginning to close (stem exclusion) and tree competition accelerates. PCT reduces the competition for the remaining trees, opens up the canopy to allow light to penetrate, and makes the stand more resistant to winds (or wind firm). PCT also produces fast growing trees with more and larger limbs, which can affect the future commercial value of the trees. Stands that are not pre-commercially thinned will grow slower and will potentially have more value if grown long enough.
The time between even-aged (clear-cut) stand harvest is called the rotation age. Stands can be grown for longer or shorter rotation ages and will produce trees with different timber characteristics that will produce different products. Stands with no thinning and longer rotations produce trees with fewer limbs and timber with clearer (knot free) wood. Rotations of 150+ years would be required to start producing wood with some OG characteristics for niche products.
Stands can be commercially thinned (CT) during the rotation period. Thinning is considered a commercial thin if some of the by-products of the thin have commercial value. Biomass utilization can potentially add value to thinning by-products. There are many types of CT prescriptions, such as spacing, thinning from below (which removes the smaller sub dominate trees), thinning from above (which removes larger trees), thinning to a specified basal area, thinning strips of specified widths, and creating openings or gaps. As with PCT, each of these prescriptions will produce stands with different timber characteristics that will produce different forest products.
YG can be managed to create uneven aged stands as well. This requires prescriptions that create small openings, either by strips or gaps in the stand and allow the establishment of younger trees. The more frequently the stand is entered to create openings the more age classes the stand will have. The Alaska Division of Forestry and The Nature Conservancy developed guidelines for a strip thinning prescription that would allow for harvest efficiency and over time convert and maintain an even-aged YG stand to an uneven-aged stand. The same guidelines could be used with gap thinning for similar results. Variable density thinning with skips and gaps is a relatively new way of treating even aged stands to both create diversity and to improve production.
The Staney Community Forest Report outlines how this group addressed YG management and prescriptions.